This invention relates to chemical sensors which detect the presence of selected air contaminants through changes in electrical impedance resulting from structural changes in a vapor-sensitive polymer. More specifically, the invention relates to sensors that can be incorporated in gas filtering devices so as to provide a warning when the life of a filter is at or near exhaustion.
Air purifying respirator cartridges are used to allow workers to remain in an ambient which contains toxic gases that would be harmful to breathe. Depending on the concentration of the toxic gas and other factors, a respirator cartridge may last from minutes to days before the adsorbent bed is expended. One of the major problems users face is determining when the service life has ended. It is an object of this invention to provide an active-end-of-service-life-indicator (AEOSLI) that could be placed in the adsorbent bed and would signal the user when the life of the cartridge was nearing the end.
The presence of an unambiguous signal warning the user that the cartridge is almost expended would give greater protection to the user, who would not have to rely on sensory indicators such as dizziness. In addition, an AEOSLI could be used for those toxic gases for which there is no sensory warning, regulations permitting. This could lead to the use of air-purifying respirators with many compounds which now require the use of a SCBA (self-contained breathing apparatus).
The development of an AEOSLI is of especially great significance today since the official threshold level values of about 500 compounds have been lowered, and adequate sensory warning is not provided at these reduced levels for many compounds. An AEOSLI will make it possible to continue to employ an air purifying respirator instead of a SCBA in many applications. Simple positive and negative pressure respirators provide the economic benefits of lower cost and increased productivity, as well as improved safety and comfort for workers. All in all, an AEOSLI offers the very important advantages of safer and more economical protection (no need to undergo sensory warning or change canisters for every shift) and of allowing the use of air-purifying respirators with compounds that don't provide sensory warning at the permissible levels. Thus, the AEOSLI could result in considerable savings to industry and in increased worker safety and comfort.
It is a major objective of this invention to provide an AEOSLI with an unambiguous alarm to warn the user when at least 10% of the respirator cartridge life remains without the AEOSLI interfering with the operation of the respirator.
It is another purpose of this invention to provide novel microchemical sensors that operate on very little power and can detect low levels of chemical vapors and gases inside an active adsorbent bed.
It is yet another object of the invention to interface the new microsensor technology with existing adsorbent-based protection systems to create a new respiratory protection device that can alert and alarm the user when the adsorbent is spent and the protection is not adequate.
It is still another object of the invention to provide small chemical sensors that are very low in cost and power consumption.
The prior art pertaining to AEOSLI devices includes U.S. Pat. No. 4,847,594, issued on Jul. 11, 1989, to J. R. Stetter, and U.S. Pat. No. 4,873,970, issued on Oct. 17, 1989, to M. Freidank et al. The patent to Freidank et al. utilizes electrochemical sensors only. Chemiresistive sensors are disclosed in Stetter's patent and also in James P. Dolan's U.S. Pat. No. 4,129,030 (Dec. 12, 1978), U.S. Pat. No. 4,224,595 (Sep. 23, 1980), and U.S. Pat. No. 4,237,721 (Dec. 9, 1980). However, none of these patents disclose chemiresistive sensors that are based on structural changes in vapor-sensitive polymers.
It is therefore still another object of the invention to provide chemical sensors in which the electrical impedance changes as a result of structural changes in a polymer upon exposure to certain vapors.